Generally, pressure reducing valves (hereinafter referred as ‘PRV’) are used in steam industry to reduce pressure of boiler steam to the point of application (process equipment). The PRV available at present performs perfectly well when pressure & flow condition (upstream & downstream) are fairly stable. However, in process industries, steam demand always varies. When process starts, pipeline & all products are cold & it is required to heat them up as fast as possible. This needs higher steam flow rates. Once, the temperature reaches the desired degrees, the demand of flow drops. Further, flow is required only to cater fluid/media heating & take care of heat losses.
In general, two PRV are provided in parallel on the pipe lines to fulfill the pressure reduction requirement. Both PRVs are initially set ‘ON’ to meet higher demands. Additionally, a smaller PRV is used to meet low flow demands.
Referring to FIG. 1 show a front view of the pressure reducing valve (1000) of the prior art. The pressure reducing valve (1000) includes main body (110), an inlet (112), an outlet (114), a stem (116), a seat (118), a pilot diaphragm (120), a main diaphragm (122), and a head (124). The pressure reducing valve (1000) works by balancing downstream pressure through a pressure sensing pipe (60) (refer FIG. 5) against a pressure adjustment control spring (126). The sensing pipe (60) provides a passage for the media between the main diaphragm (122) and the pilot diaphragm (120). The required amount of pressure is adjusted by adjusting the control spring (126). The main diaphragm (124) is larger as compared to the pilot diaphragm (120). Upon reaching a desired pressure, the pressurized media pressurizes the pilot diaphragm (120) and the main diaphragm (122). As the area of the main diaphragm (122) is more, as compared to the area of the pilot diaphragm (124) mechanical advantage is achieved and offers accuracy with small proportional offset, therefore the main diaphragm (122) moves upwards.
The upward movement of the main diaphragm (122) moves the stem (116) upward, thereby lifting the head (124) away from the seat (118) for enabling the media to egress out from the outlet (114) to modulate a control pressure. Further, under stable conditions, pressure below pilot diaphragm (122) balances force against control spring (126), and the pressure transferred below main diaphragm (122) is also constant, therefore balanced condition is achieved.
When downstream pressure rises, pressure below the pilot diaphragm (120) is greater than force created by the control spring (126), the pilot diaphragm (120) pushing a pilot seat to closer condition. This interrupts pressure below main diaphragm (122). As there is always a pressure on top of main diaphragm (122) (downstream pressure) & now this is more than the pressure under the main diaphragm (122), this transfers the pressure through control pipe (60) & surplus pressure orifice, thereby balancing pressure therebetween.
Further, ratio of maximum to minimum controllable flow is called “rangeability” of the PRV. If one oversized PRV is used instead of two (one small & one large), then phenomenon called hunting occurs. Hunting means the valve opens to cater to the flow but the demand being very low tries to shut down again and this cycle continues.
With this hunting, the head (124) of the PRV (1000) will try to come closer to the seat (118) while passing wet steam causing wiredrawing & premature erosion which will reduce life of the head (124) and the seat (118). In addition, any small movement of oversized head will produce a relatively large change in flow of the PRV (1000) making it more difficult to control accurately. In short, range-ability of the PRV (1000) is limited. When the PRV (1000) is used in this condition, all parts reach their fatigue life very quickly and fail.